The present invention relates to a monolithically integrated control circuit for a brushless or commutatorless DC motor comprising
a commutation signal source controlled by rotational motor position signals,
a driver circuit controlled in accordance with the commutation signals and serving for applying driving pulses, which result in a rotating magnetic field, to the motor windings, with one driver stage being provided for each motor winding phase,
and a pulse shaping circuit effecting sloping of the driving pulse edges.
Such a control circuit is suited for the control of brushless DC motors that must be controlled very exactly. A microcomputer is preferably used for motor control. Such motors are typically employed as head wheel motors of video recorders and driving motors for record players, CD players (compact disk players) and magnetic disk units, e.g. floppy-disk drivers.
Driving pulses are produced at the driver outputs of a control circuit for such a motor in such a sequence that a rotating magnetic field is created in the motor winding or coil. The time position of the individual driving pulses defines the commutation sequence, and the amplitude of the driving pulses defines the motor output.
A typical servo system for a four-phase brushless motor is shown in FIG. 1. It comprises the motor 1, a motor controller 3 as well as a control circuit 4. The motor 1 is provided with a position sensor P.sub.S and with a speed sensor G.sub.S supply rotational rotor position signals and, respectively, motor speed signals to the controller 3 via connecting lines 2. The controller 3 derives from the rotational motor position signals and from the motor speed signals an analog control signal 6 as well as commutation signals 5. These signals are fed to a signal processing circuit S.sub.V belonging to the control circuit 4. The signal processing circuit S.sub.V generates driver control pulses 12 and switching control pulses 14 which are fed to a driver circuit T.sub.S that also belongs to the control circuit 4. Four outputs 16A to 16D of the driver circuit T.sub.S are connected to winding terminals A to D of the motor 1. Between the controller 3 and the driver circuit T.sub.S there is provided a special function circuit S.sub.F effecting, for instance, a general release or a braking function.
Conventional control circuits produce at the driver outputs 16A to 16D rectangular driving pulses causing in the windings current pulses having a current path that depends on the amplitude of the driving pulses, the winding inductance and on the motor voltage (emf voltage). The rapid voltage and current changes at the driver outputs, caused by the rectangular driving pulses, lead to kick-back pulses or "flyback pulses" which are caused by a sudden magnetic discharge of an abruptly turned-off inductor as it is generally known.
Typical voltage and current patterns at the outputs of the driver circuit of a conventional control circuit are shown in FIG. 5a. V.sub.OA and V.sub.OB are the voltage patterns, and I.sub.OA and I.sub.OB are the current patterns at the driver outputs 16A and 16B. The current and voltage patterns belonging to driver output 16A are shown in full lines, and the current and voltage patterns belonging to driver output 16B are shown in the form of broken lines. The curves V.sub.MA and V.sub.MB represent the associated motor voltage patterns.
FIG. 5a shows the example for a driver circuit designed in "tri-state technology", i.e. a driver circuit having tristate outputs. For instance, the driver output 16A is in the H (High) state during the commutation clock period T.sub.1, in the L (Low) state during the commutation clock period T.sub.3 and in the open or high-impedance state during the remaining two commutation clock periods T.sub.2 and T.sub.4. The same holds for the driver output 16B, but with a time shift of one commutation clock period.
FIG. 5a clearly indicates the strong flyback pulses occurring when abruptly switching the driver outputs from the H state or L state, respectively, into the tri-state condition.
The flyback pulses cause problems. On the one hand, they cause a loud motor noise that is disturbing for the user of a video recorder for instance. On the other hand, due to their high frequency content the flyback pulses cause strong electromagnetic radiation. This may lead to considerable disturbances in the apparatus equipped with the motor as well as in other apparatus. In the case of a head wheel motor for a video recorder, the magnetic heads disposed on the head wheel are located in close proximity to this source of disturbing radiation.
In order to counteract such disturbances, a known control circuit for a brushless motor has an RLC filter connected between each driver output and the associated winding terminal of the motor. By means of these filters it is possible to slope the edges of the driving pulses reaching the motor winding. In this manner one can avoid the occurrence of electromagnetic disturbances on the lead wires to the motor windings as well as on the motor windings. However, due to the inductances of the RLC filters flyback pulses are still present at the driver outputs of the control circuit. Thus, disturbing electromagnetic radiation still occurs at the site of the driver outputs. The advantage of having disturbing radiation no longer at the site of the motor and on the motor lead wires, but only at the driver outputs, can be achieved only with the great circuit expenditure of providing one RLC filter for each driver output.